The present invention relates to spectrometry, and more particularly, to planar spectrometer devices that enable analysis of compounds by high field asymmetric waveform ion mobility techniques, and method of achieving same.
A chemical sensor system provides samples to a detector, i.e., a spectrometer, for identification. The device may take samples directly from the environment, or it may incorporate a front end device to separate compounds in a sample before detection.
In making such measurements, whether in the lab, the workplace or in the field, there is a need for unambiguous compound identification. One approach is to employ a combination of instruments capable of providing an orthogonal set of information for each chemical measurement. (The term orthogonal will be appreciated by those skilled in the art to mean data which enables accurate identification of a particular chemical species, and uses a different property of the compound for identification.)
One combination of known instruments is a gas chromatograph (GC) attached to a mass spectrometer (MS). The GC separates compounds in a gas sample that improves the chemical identification capability of the spectrometer. The mass spectrometer is generally considered one of the most accurate detectors for compound identification. A mass spectrometer can generate a fingerprint pattern of fragment ions based on mass corresponding to each compound eluting from the GC. Use of the mass spectrometer as the GC detector dramatically increases the value of analytical separation provided by the GC. The combined GC-MS information, in most cases, is sufficient for unambiguous identification of the compound.
Mass spectrometers are expensive, easily exceeding $100 K, and are difficult to deploy in the field. Mass spectrometers also suffer from the need to operate at low pressures resulting in complex systems, and their spectra can be difficult to interpret often requiring a highly trained operator. As well, the GC-MS is not well suited for small, low cost, fieldable instruments.
Lower cost and compact, reliable instrumentation for the laboratory and field is a desirable goal. In the lab, there is a continuing demand for improvements in affordable bench top analytical equipment. As well, there is a developing interest in making in situ measurements of chemicals present in complex mixtures at industrial and environmental venues. Therefore the search continues for low cost, high quality, and compact chemical detector equipment.
Time-of-flight Ion Mobility Spectrometers (TOF-IMS) have been described as functional detectors from early in the development of ion mobility spectrometry. High-speed response and low memory effects have been attained, and the gas phase ion chemistry inside the TOF-IMS can be highly reproducible providing the foundation to glean chemical class information from mobility spectra. Widespread use still remains a problem for TOF-IMS. Despite advances over the past decade, TOF-IMS flow channels (also referred to as drift tubes) are still comparatively large and expensive and suffer from losses in detection limits when made small.
The high field asymmetric waveform ion mobility spectrometer (FAIMS) is an alternative to the TOF-IMS. In a FAIMS device, a gas sample that contains a chemical compound is subjected to an ionization source. Ions from the ionized gas sample are drawn into an ion filter and subjected to a high field asymmetric waveform ion mobility filtering technique. Select ion species allowed through the filter are then passed to an ion detector, enabling indication of a selected species.
The FAIMS filtering technique involves passing ions in a carrier gas through strong electric fields between the filter electrodes. The fields are created by application of an asymmetric period voltage (typically along with a further control bias) to the filter electrodes.
The process achieves a filtering effect by accentuating differences in ion mobility. The asymmetric field alternates between a high and low field strength condition that causes the ions to move in response to the field according to their mobility. Typically the mobility in the high field differs from that of the low field. That mobility difference produces a net displacement of the ions as they travel in the gas flow through the filter. In absence of a compensating DC bias signal, the ions will hit one of the filter electrodes and will be neutralized. In the presence of a specific DC bias signal, a particular ion species will be returned toward the center of the flow path and will pass through the filter without neutralization. The amount of change in mobility in response to the asymmetric field is compound-dependent. This permits separation of ions from each other according to their species, in the presence of an appropriately set DC bias.
In the past, Mine Safety Appliances Co. (MSA) made an attempt at a functional FAIMS implementation in a cylindrical device with coaxial electrodes, such as disclosed in U.S. Pat. No. 5,420,424. (This FAIMS technology is referred to by MSA as Field Ion Spectrometry (FIS), see FIG. 1.) The device has been found to be complex, with many parts, and somewhat limited in utility.
A characteristic of known coaxial FAIMS devices is the relatively slow detection time. This can be a serious problem when coupling to a prefilter or separator, such as a GC. A GC operates so rapidly that known FAIMS devices cannot generate a complete spectra of the ions present under each GC peak. Therefore these prior art FAIMS devices would have to be limited to a single compound detection mode if coupled to a GC, with a response time of about 10 seconds. Any additional compound that is desired to be measured will take approximately an additional 10 seconds to measure. A FAIMS device with faster response times is much desired.
While the foregoing arrangements are adequate for a number of applications, it is still desirable to have a low cost and compact spectrometer that can render real-time or near real-time indications of detected chemical compounds, whether for the laboratory, the battlefield or in other environments, and whether as a stand alone detector or in cooperation with other devices such as a GC or an MS.
It is therefore an object of the present invention to provide a functional, small, spectrometer that overcomes the limitations of the prior art.
It is a further object of the present invention to provide a chemical sensor that features the benefits of FAIMS but is able to operate rapidly, affording real-time or near real-time detection.
It is a further object of the present invention to provide a chemical sensor that features the benefits of FAIMS and is able to detect multiple species simultaneously.
It is a further object of the present invention to provide a chemical sensor that features the benefits of FAIMS but is able to detect positive and negative ions simultaneously.
It is a further object of the present invention to provide a chemical sensor that features the benefits of FAIMS but is able to be part of a system that generates orthogonal data that fully identifies a detected species.
It is a further object of the present invention to enable a new class of chemical sensors that can rapidly produce unambiguous, real-time or near real-time, in-situ, orthogonal data for identification of a wide range of chemical compounds.
It is a further object of the present invention to provide a class of sensors that enable use of pattern recognition algorithms to extract species information.
It is a further object of the present invention to provide a class of sensors that do not require consumables for ionization.
It is a further object of the present invention to provide a class of sensors utilizing an arrays of FAIMS devices each tuned to detect a particular compound, such that multiple compounds can be detected rapidly, with simplified electronics.
It is a further object of the present invention to provide a class of sensors utilizing arrays of FAIMS devices to provide redundancy in ion detection.
It is a further object of the present invention to provide a class of sensors utilizing arrays of FAIMS devices where each ion filter has its own flow path (or flow channel) and is doped with a different dopant for better compound identification.
It is a further object of the present invention to provide a class of detectors that can provide information on the cluster state of ions and ion kinetics by varying the amplitude of the high voltage asymmetric electric field.
It is a further object of the present invention to provide a class of detectors that can provide information on the cluster state of ions and ion kinetics by adjusting the frequency of the asymmetric signal.
It is a further object of the present invention to provide a class of detectors that can provide information on the cluster state of ions and ion kinetics by adjusting the flow rate of ions through the device.
It is a further object of the present invention to provide a class of detectors that can provide information on the cluster state of ions and ion kinetics by varying the amplitude of the high voltage asymmetric electric field, or by adjusting the frequency of the asymmetric signal, or by adjusting the flow rate of ions through the device, or any combination of these techniques.
It is further an object of this invention to provide a class of sensors that can quantitatively detect samples over a wide range of concentrations through controlled dilution by regulating the amount of ions injected into the ion filter region by controlling the potentials on deflector electrodes.